Precipitation-dissolution behaviors of rare earth ions in H3PO4Ca(H2PO4)2 solutions

Abstract

Abstract In order to achieve deeper understanding of rare earth elements (REEs) behaviors during phosphate rock processing with H3PO4. The solubility of REEs in Ca(H2PO4)2 H3PO4 solutions with various concentrations of Ca(H2PO4)2 at different temperatures were tested. The results demonstrate that REEs solubility decreases sharply with the increasing concentration of Ca(H2PO4)2. Equations between [REE3+] and [H+], [H+] and [Ca2+] in Ca(H2PO4)2 H3PO4 solutions were built based on the precipitation-dissolution equilibrium of rare earth phosphates and the ionization equilibrium of H3PO4. According to the equations, the decreasing mechanism of REEs solubility caused by elevated concentration of Ca(H2PO4)2 was determined. The mechanism can be illustrated as that the elevated concentration of [H2PO4–] decreases the concentration of hydrogen ion by retarding the ionization process of H3PO4 and directly promotes the precipitation of rare earth phosphates. Furthermore, it can be easy deduced that similar effect would be caused by the other cation impurities (Fe3+, Al3+, etc.) on REEs solubility based on the mechanism. In addition, superimposed reduction effect on REEs solubility caused by the elevated concentration of Ca(H2PO4)2 and the elevated temperature is found. This superimposed effect leads to a super low solubility of REEs in Ca(H2PO4)2 H3PO4 solution. On the basis of the experimental study, outlooks and suggestions for further development of REEs recovery method are given.